@Pirots, I thinks a sensible question first is do you have any soldering skills and knowledge of Ohms law along with a multimeter?
It is very possible the batteries within the pack are not completely dead and dropped to around 2.6V. It is possible to “jump start” the batteries but you would need a variable output power source where you set the voltage for the pack at 25.2V but up the current so the batteries would begin to charge and then allow your normal charger to takeover in the hope they recover. For safety reasons given the size of the pack an understanding of Ohms law would be preferable and then theres the cost of a variable power source or a larger charger, which could be money down the pan especially if the batteries are below 2volts. Use of a multimeter could determine the average voltage of each battery in the pack.
You could refurbish your current pack by replacing the individual 18650 batteries but there would be a fair amount of de soldering and soldering involved, ensuring you don’t get any dry joints or apply to much heat to the batteries whilst doing so.
Even if you bought a generic a generic 7s6p battery pack there would be some soldering, to solder fly leads on etc and the precautions with refurbishing your current pack would need to be taken into account.
You could ask an electrician or electronics engineer to refurbish your current one or fit a generic 7s6p and do all the soldering for you. But it could prove costly this way unless you new someone.
Its not difficult to do, but its easy for me to say that as its been my trade for nearly 40yrs. If your confident and would like to have ago and have expectable skills of soldering and an some understanding of Ohms law I’d be willing to guide you through the options. I only say this for your own safety as there are risks if your not sure 🙂
Putting all that aside, given you don’t kayak very often would something like these be suitable for your needs. Admittingly they may not be as powerful. They would also more than likely work out cheaper than repairing yours.